The present invention relates generally to ways to increase humidification of fuel cell reactants, and more particularly to ways to integrate water transfer functions into anode or cathode flowfield plates to facilitate such humidification.
In many fuel cell systems, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. An appropriate catalyst (for example, platinum) ionizes the hydrogen into a proton and electron on the anode side such that upon subsequent combination of the proton with oxygen and the electrons at the cathode side, electric current is produced with high temperature water vapor as a reaction byproduct.
In one form of fuel cell, called the proton exchange membrane (PEM) fuel cell, an electrolyte in the form of a membrane is sandwiched between two electrode plates (typically in the form of a porous diffusion media) that make up the anode and cathode. This layered structure is commonly referred to as a membrane electrode assembly (MEA), and forms a single fuel cell. Many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. While the present invention is especially applicable to the PEM fuel cell, it will be appreciated by those skilled in the art that the use of other fuel cell configurations with the present invention is also within the purview of the present disclosure.
Fuel cells, particularly PEM fuel cells, require balanced water levels to ensure proper operation. For example, it is important to avoid having too much water in the fuel cell, which can result in the flooding or related blockage of flowfield channels, thereby preventing the flow of reactants. Contrarily, too little hydration limits electrical conductivity of the membrane and can lead to premature cell failure. Exacerbating the difficulty in maintaining a balance in water level is that there are numerous conflicting reactions taking place in a fuel cell that are simultaneously increasing and decreasing local and global hydration levels. In addition to the flooding mentioned above, water can be dragged from the anode and into the cathode by the ionized protons moving from the anode. This phenomenon, known as electro-osmotic drag, significantly contributes to the removal of water molecules from the anode. Other mechanisms may also be prevalent, including diffusion of water molecules from the cathode to the anode across the membrane due to pressure differences.
Furthermore, many of these effects are localized such that even if global levels of hydration are maintained in the fuel cell, there is no guarantee that local water balance is maintained. One potential method of ensuring adequate levels of hydration throughout the fuel cell includes humidifying one or both of the reactants before they enter the fuel cell. For example, the water produced at the cathode can be used, with appropriate humidification devices, to reduce the likelihood of anode or membrane dehydration. These humidification devices can involve either external supply means (for example, a separate water reservoir) or self-supply means, such as a water vapor transfer device that extracts the moisture from a humid fuel cell exhaust flowpath and places it into a reactant feed path.
Unfortunately, the use of humidification devices (and their supporting control systems) necessitates additional fuel cell system weight, size and complexity, as well as reductions in fuel cell output or efficiency in situations where such componentry requires a source of power to operate. Such disadvantages are especially troublesome for vehicle-based fuel cell applications, as the often redundant componentry would take up precious vehicle space otherwise used for passenger, comfort or safety features. Accordingly, there exists a need for an integrated approach to hydrating a fuel cell to avoid the cost, complexity or operability associated with traditional approaches.